Conventional Hall Effect ion source and plasma systems typically include a plasma accelerator, a gas distributor for introducing a gas into the plasma accelerator, and an anode located at one end of a channel. A DC voltage provided by a DC power source connected to an electric circuit creates an electric potential between the anode and a floating externally located cathode that emits electrons. A magnetic circuit structure with a magnetic field source, e.g., one or more permanent magnet or electromagnetic coil, creates a transverse magnetic field. The electric circuit and the magnetic circuit structure establish an axial electric field. The transverse magnetic field presents an impedance to flow of electrons attracted to the anode. As a result, the electrons spend most of their time drifting azimuthally (orthogonally) due to the transverse magnetic field. The result is the electrons collide with and ionize the neutral atoms in the propellant or gas. The collisions create positively charged ions in the gas to create plasma. The ions are accelerated by the axial electric field to create an ion flux that may be used, inter alia, to create thrust. See e.g., U.S. Pat. Nos. 6,150,764, 6,075,321, and 6,834,492 and U.S. patent application Ser. No. 11/301,857 filed Dec. 13, 2005, all by one or more common inventors hereof and the same assignee, and are incorporated in their entity by reference herein.
Conventional Hall Effect ion source and plasma accelerator systems rely on the DC voltage provided by the DC power source connected to the electric circuit in order to determine the strength of the axial electric field and therefore the acceleration and energy level of the ions in the plasma. The DC voltage level also affects the flow and energy level of electrons attracted to the anode and therefore the ionization of the gas to create plasma. The result is ionization and acceleration are closely coupled causing the system to have a smaller operating envelope and lower efficiency than may be possible if the processes could be separated. Coupling acceleration and ionization prevents separately “tuning” the ion energy level, the amount of ionization provided by the system, and the total flux of the ions. Therefore, conventional Hall Effect ion source and plasma accelerator systems are unable to efficiently generate ion flux with ions having low (e.g., <10 eV) or mid ion energy (e.g., <130 eV) levels while maintaining a constant high ion flux density.
Conventional Hall Effect ion source systems are also limited by the maximum DC voltage that can be utilized because arcs are typically generated in the discharge region of the plasma accelerator at high DC voltages, typically greater than about 1,000 V. This limits the maximum DC voltage that can be employed and therefore the maximum specific impulse that can be achieved.